3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Investigation of the effects of zinc oxide nanoparticles and synthesized cellulose nanocrystals (CNCs) on emulsion-based drilling fluids(2019) Aka, Tiemele Wilfried AndersonDrilling Mud holds an important role in the drilling process in such a way that it is a determinant key to the success of the operation as well as the money spent throughout the process. Indeed the success and the cost of the operation can be severely impacted by some challenges experienced while drilling such as temperature and pressure conditions which leads to fluid loss, fluid deterioration...As a result there is a need to formulate a fluid with desirable rheological properties to withstand such undesirable parameters. Therefore this work was aimed to improve emulsion drilling fluids (EDFs) based nanoparticles with enhanced properties. Many investigations were performed to find a proper emulsion stability as well as a good drilling fluid performance. The stability of the prepared emulsion drilling fluids was done using surfactant with different concentrations for several days. After several days of preparation, the EDFs containing DTAB as surfactant have showed a better emulsion stabilizer compared to the Triton X-100 ones. In addition an investigation combining both NPs and surfactants confirmed the used of NPs to improve DF and revealed the effective use of ZnO NPs for drilling fluids application and preferentially with DTAB as surfactant. Following that result, the 2nd part of the work was based on the synthesis and characterization of CNCs as NPs to formulate EDF with DTAB as surfactant. The CNCs NPS were successfully obtained via the method of oxidation of microfibrillated cellulose through TEMPO-mediate and after characterization using TEM, spherical NPs with small size varying from 10-50nm were observed. The FANN® Model 35 viscometer served to display the behavior of the shear stress and viscosity of the prepared fluids against variable shear rate at variable NPs and temperature concentration. The rheological and filtration properties were increase with increase in CNCs content from 0.8 to 1.2% of fluid in room temperature and with an increase in temperature.Item Experimental and Numerical Characterization of Interlaminar Properties of SWCNTs Doped PAN Nano-mats Strenthened Multiscale Hybrid Composites(2019) Arif, MuhammadPolymer composite reinforced with conventional macro size bres have taken a major role in various modern engineering applications and their demand is ever increasing due to their light weight and design exibility. Various im-provements in manufacturing methods, fabrication techniques and composite constituents have been made over the years to produce better polymer com- posites. However, the major challenge of the conventional polymer composites is that failure at the matrix rich interlaminar region still remains and limits their performance resulting in unreliable usage of these composites. A number of research attempts had little success due to various limiting factors have been carried out to rectify this problem. One of the potential methods expected to improve the strength of the interlaminar region is the incorporation of nano-size llers, such as electrospun Polyacrylonitrile (PAN) nano bres mat as an interalia into the matrix rich interlaminar region of the conventional polymercomposite. However, controlling the alignment and distribution of the PAN nano bres during the chaotic electrospinning process is a major hurdle for im-proving the interlaminor regions. The random orientation of electrospun PAN nano bres mat reduces their strengthening e ect and also the required material properties.Hence, the current study has focused on the design of electrospinning process for improving the orientation and distribution of the PAN nano bre mats. The developed electrospinning process was used to produce random and aligned PAN nano bres mat and also used for producing both pristine and function-alized single walled carbon nanotubes (SWCNTs) doped PAN nano bres mat. These nanomats were then sandwiched with the glass bre-epoxy matrix to produce nano strengthened multiscale hybrid composites. As part of the electrospinning procedure, electric elds of general electrospin- ning technique were manipulated using two position adjustable auxiliary ver- tical electrodes (AVEs) to produce aligned nano bres mat along with reduced diameter. So as to optimise the electrospinning parameters, the e ect of AVEs on the PAN nano bres mat orientation, distribution and diameter were ex- perimentally matrixed and analysed. The fractographic study showed that auxiliary vertical electrodes (AVEs) added to the electrospinning process re- duced the diameter, enhanced the alignment of the nano bres and improved molecular orientation. Among four di erent volume fractions of 0.1%, 0.2%, 0.5% and 1% randomly oriented PAN nano bre mats, the volume fraction of 0.5% PAN polymer was selected to manufacture aligned PAN nano bres mat strengthened hybrid composite based on the improved experimental randomly oriented PAN nano bre mats strengthened hybrid composites. A series of tests showed that glass bre composites (GFC) reinforced with the volume fraction of 0.5% aligned nano bre mats were better than those of 0.5% randomly distributed nano bre mats. The aligned nano bre mat with reduced diameter increased the tensile, exural, and impact properties of glass bre composite by 68.91%, 95.32% and 45.30% respectively. Aligned nano bres mat was further utilised to align and disperse the pristine and functionalized SWCNTs into the interlaminar region of breglass compos- ite. Alignment and a nano-range diameter of nano bres helped in improved distribution and alignment of pristine SWCNTs (p-SWCNTs), which was re- ected in an increase in tensile, exural and impact resistance by 89.30%, 105.48% and 107.17% respectively. A nondestructive functionalization method (Friedel craft alkylation) was used to improve the interface bonding of SWC- NTs with the host PAN polymer nano bre. PVA chains crafted to the surface of the SWCNTs without damaging the wall was con rmed using FTIR and Ra- man spectroscopy. The e ect of functionalized SWCNTs (f-SWCNTs) doped aligned PAN nano bre mats improved the properties of nano-hybrid multiscale composite up to 111.34%, 117.11% and 180.03% in tensile, exural and impact resistance respectively. A multiscale model was used to determine the properties of the multiscale nanohybrid composite strengthened with random and aligned PAN nano bre mats, PAN doped with p-SWCNTs and f-SWCNTs aligned nano bre mats. Three length scales, such as nano, micro and macro scales were modelled. At rst, a numerical model was developed to determine the elastic properties of di erent carbon nanotubes (CNTs) i.e. Pristine and defective single wall (SWCNTs), double wall (DWCNTs), and multiwall (MWCNTs) for zigzag and armchair con gurations. CNTs atomic geometry was replicated with an equivalent space frame structure (SFS). Co-ordinates de nition of SFS of CNTs was developed in MATLAB code and transferred to the nite element analysis (FEA) software 'ANSYS'. The basic entity of SFS, C-C chemical bond was designed as a circular beam of orthotropic properties. The properties were determined by linking the energy equation of molecular mechanics to structural mechanics along with the parametric study. The van der Waals forces between inter-shell of DWCNTs and MWCNTs were modelled as linear elastic springs in a simpli ed way. The simpli ed model avoided the problems due to the nonlinear behaviour of van der Waals forces and improved the performance of the FEA software by computational resources. The e ect of chirality, vacancy defects, di erent diameters and numbers of walls on elastic properties of CNTs were calculated, tabulated and compared with each other. The result of the proposed SFS model with orthotropic properties was compared with others result. The SFS model is found better than the equivalent shell model as the defects can be placed at the exact location and a more realistic behaviour could be predicted. The SFS models could be developed with any type of defects, a number of walls, van derWaals and agglomerated forms with variable geometries. Using the space frame structures (SFS) of SWCNTs, the nanoscale RVE of PAN nano bre doped with SWCNTs was modelled. Simulated results of nanoscale RVEs were used to determine the equivalent properties of p-SWCNTs and f-SWCNTs doped nano bres, which were further used in microscale RVE models. Four micro scale RVEs were developed to represent the random and aligned PAN nano bres mats, PAN nano bres mat doped with p-SWCNTs and f-SWCNTs aligned PAN nano bres mat in epoxy matrix. Analysed micro scale RVEs provided equivalent properties of interlaminar regions developed with random and aligned PAN nano bres mat, PAN doped with p-SWCNTs and f-SWCNTs aligned nano bres mat. At the macro scale, MNHCs were de- veloped with equivalent interlaminar regions and analysed. The results of the simulated MNHCs were compared with experimentally obtained results. The results of the experimental study suggested that aligned nano bres with reduced diameter improved the properties of interlaminar region noticeably than the random nano bres mat of the same volume fraction. Aligned nano - bres successfully placed the pristine and functionalized SWCNTs within the multiscale hybrid composites which signi cantly improved the properties of the multiscale hybrid composite. Results of the multiscale modelling were in line with the experimental results, which could be useful in extending the small- scale theoretical results to the real-life applications.Item A parametric study of resin-gel synthesis to understand the formation mechanism of titanium-oxide nanoparticles(2018) Narrandes, Ashvir AshwinThe relatively unknown resin-gel synthesis technique has the potential to form multi-mode mixed metal oxide nanoparticles with differing stoichiometries. These oxides can be employed in a plethora of applications. In order to exploit these benefits, the mechanism of nanoparticle formation must be understood. To this end, this study embarked on a parametric investigation to gain insights on the formation of the less stable anatase and more stable rutile (titanium dioxide) using resin-gel synthesis. By adjusting parameters such as the type of polymer, solvent, acid, and metal ion precursor, and by varying other parameters such as the polymer chain length, polymer stoichiometry, and heating rate, a model for nanoparticle formation was developed and refined. This model considered the formation of hydroxylated metal ion species following the addition of a metal ion precursor to a hydroxyl-containing solvent. These species were protected and stabilised by the remaining fragments of solvent components. In addition, the size of the ligands attached to the metal ion precursor governed the amount of protection and stabilisation afforded to the hydroxylated species by the precursor. These complexes were coordinated to polymer chains that underwent degradation during the course of heating and ignition. Polymer degradation produced polymer reaction chambers. The formation, action, and interaction of these chambers with developing titania crystallites are a novel finding of this work. The sizes of these chambers were controlled largely by the quantity of polymer present in the reaction. The number of accessible oxygen sites on the precursor determined the degree of association between the metal ion complexes and the reaction chambers. If the association was intimate, the polymer reaction chambers served to stabilise and protect the newly nucleated anatase particles. If the combination of protection effects afforded by the solvent components, precursor ligands, and association of reaction chambers of appropriate sizes was insufficient to stabilise nucleated anatase, it readily converted into the rutile phase. Anisotropic growth along [0 0 1] then caused rutile to form nanorods. Rutile mesocrystals developed following sufficient polymer degradation. The association of nanoparticles with polymer fragments was viewed using TEM. Additionally, TEM investigations revealed the presence of polymer-derived superstructures containing reaction chambers. Reaction chambers presented with various morphologies and were composed of crystalline carbon.Item Surface enhanced raman scattering dependence on chain-length of 1-alkanethiols on gold and silver nanoparticles(2018) Shumbula, Ndivhuwo PrinceSurface enhanced Raman spectroscopy (SERS) is a surface sensitive technique through which the Raman signal of molecules adsorbed on a metallic surface is enhanced. It is a molecular vibrational technique which has evolved from the classic Raman spectroscopy in early 1970s. This technique is widely used for identification of solid, liquid and gas analytes. It also enables a sensitive detection of single molecule and also provides its distinguished chemical fingerprints. This technique has the ability to improve the Raman cross section by orders of up to 1015. The enhancement of local magnetic field due to localised surface plasmon resonance (SPR) is greatly dependent on SERS substrates (i.e. metal nanoparticles) used and the molecule (Raman reporter) attached to the substrate. In the current project, we report on the effect that the chain-length of 1-alkanethiol functionalized on gold and silver nanoparticles (AuNPs and AgNPs) has on the Raman scattering enhancement. AuNPs and AgNPs were synthesized by the reduction of chloroauric acid and silver nitrate respectively, using tri-sodium citrate as a reducing agent. Citrate capped nanoparticles were obtained and further functionalized with 1-alkanethiols of different chain-lengths, i.e. pentanethiol (PT), decanethiol (DT), dodecanethiol (DDT), and pentadecanethiol (PDT). The 1-alkanethiols were chosen as Raman reporters because they are able to form self-assembled monolayer (SAM) systems on the surface of metals, thus influencing their stability. The optical properties of both functionalized (SAMs) and unfunctionalized AuNPs and AgNPs were studied using ultraviolet-visible (UV-Vis) spectroscopy. A red shift of the SPR bands of AuNPs and AgNPs prior to functionalization with 1-alkanethiols was observed from the UV-Vis spectra. AuNPs and AgNPs which were monodispersed and spherical-like morphology with the average diameters of 14 and 25 nm respectively were obtained as was evidenced from the transmission electron microscopic (TEM) analysis. The obtained negative zeta potential indicated negatively charged surfaces for both AuNPs and AgNPs. AuNPs were more stable and well dispersed in the colloidal solution as compared to AgNPs since they possessed a strong negative zeta potential. The effect of chain-length on Raman scattering was evaluated using Raman spectroscopy and the enhancement factor (EF) was calculated from the intensities of symmetric stretch vibrations of C-H observed in the region of about 2900 to 3000 cm-1 in all SERS spectra. SERS spectra for all 1-alkanethiols (Raman reporters) showed more intense characteristic peaks as compared to their classical Raman spectra. Some vibrational modes which were not observed in classical Raman spectra where observed from the SERS spectra. The shorter chain-length PT possessed a higher enhancement factor (EF) of the Raman cross-section as compared to the longer chain-lengths 1-alkanethiols. DFT and Molecular Dynamic studies were done to establish the influence of the chain length on the EF. The geometry of the RR adsorbed on the metal surface as well as the position in which the RR was adsorbed on the metal surface was found to influence the charge density transfer hence the SERS spectra obtained. From the calculations, it was evident that the adsorption of the RRs on both Ag and Au metals resulted on an enhanced Raman spectra however other factors influenced the observed EF trend.Item Tailored synthesis of shaped carbon nanomaterials over supported Au, Ni, La and La-Ni novel radially aligned nanorutile: characterisation and investigation of properties(2017) Mutambara, Farai Dziike;Radially aligned nanorutile (RANR), in the form of microspheres, were synthesised using a hydrothermal method. Parameters such as reaction time, reaction temperature, reactant concentrations, as well as the solution’s pH value result in RANR at various stages of formation. These consisted of densely packed and spherically aligned rutile phase nanorods which were oriented on a common crystallographic axis (110) that had independent terminating ends. The average diameter of these nanorods was between 2–12 nm, while their lengths ranged from 50–650 nm. Loadings ranging from 0.5 to 10 wt. % of metal nanoparticles of: Au, Ni, La and La-Ni onto the RANR support were achieved by using a deposition-precipitation method with urea. The choice of the metals was carefully done considering their positions on the periodic table. Ni is known to catalyse the synthesis of CNMs. Its behaviour was tested on the unique RANR support and compared to a higher transition metal, Au. Further testing was taken to a different metal group, La and effect of co-loading tested on La-Ni lanthanide-transition system. These metal nanoparticles (i.e. Au, Ni, La and La-Ni) that were deposited on RANR support were found to range in size from 2–3 nm, 1–6 nm, 2–8 nm and 2–10 nm respectively. The PXRD pattern of the supported catalysts predominantly exhibited peaks of pure rutile with a major peak at 2θ value of 32.5° for the major rutile peak, and minor peaks at 42.5, 48, 52.1, 64.0, 66.5, 75.0 and 82° respectively. La showed major peaks occurring at 2θ values of 30° and 15° with minor peaks at 26.5° and 28.0°. Ni had peaks at 2θ values 28°, 38.5°, 58° and 76°. Au had peaks occurring at 2θ values 24.3°, 28°, 43.5° and 48°. The peak positions of both La and Ni occurred at their respective 2θ values in the co-loaded La-Ni/RANR catalysts. TPR analyses revealed different reduction temperatures that increased with increase in wt. % loading. Au, Ni and La had mean reduction temperatures of 90 °C, 500 °C and 600 °C respectively. However, the co-loaded La-Ni/RANR catalyst showed different reduction temperatures ranging 500 – 700 °C for loading of 1 – 10 wt. %. BET surface area analysis showed the Metal/RANR catalyst decreased from 50 to 33 m2 g-1 with increase in wt. % loading from 5 – 10 wt. %. This implied that the metal particles were occupying the pore sited on the surface of the RANR support thereby decreasing the BET surface of the Metal/RANR catalyst with increase in metal wt. % loading. Parametric studies for the synthesis of shaped carbon nanomaterials (SCNMs), in particular carbon nanofibers (CNFs), via chemical vapour deposition (CVD) using La, Au, Ni and co- loaded La-Ni nanoparticles supported on RANR were performed under varying conditions including: wt. % loadings, temperature, flow rate, time and the type of metal used. The synthesis of CNFs from the decomposition of acetylene and hydrogen (H2/C2H2) was targeted for the tailored synthesis of shaped carbon nanomaterials over supported Au, Ni, La and La-Ni on novel radially aligned nanorutile. Characterisation of the various metal/RANR catalysts and the CVD products was performed by: transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), powder X-ray diffraction spectroscopy (PXRD), electron probe microanalysis (EPMA), laser Raman spectroscopy (LRS), thermogravimetric analysis (TGA), ultraviolet–visible spectroscopy (UV-Vis), Fourier transform infra-red spectroscopy (FTIR) and Brunauer-Emmett-Teller (BET) surface area measurements. La/RANR supported the synthesis of straight chain CNFs. The CNFs grew predominantly from the tips of the La nanoparticles in the catalyst. The La nanoparticles were observed on most of the tips of the CNFs suggesting that tip-growth was the mechanism of growth of the CNFs synthesised over La/RANR. During CNFs growth, the La nanoparticles were displaced from their positions on the RANR support. The dislodged La nanoparticles had a tendency of undergoing sintering to form large sized nanoparticles. Consequently, the sintered La nanoparticles catalysed the synthesis of thick CNFs with diameters ranging from 40–270 nm as compared to supported La nanoparticles with an average particle size of 6 nm. It was observed that the morphology of the La metal nanoparticles played a critical role in the control of the morphology of the CNFs and is closely related to the properties of the metal particles, the reaction conditions and the pre-treatment of the La/RANR catalyst. At 5 wt. % loadings, the average diameter of the CNFs synthesized by the La/RANR catalysts was 40 nm while the lengths ranged from 200 nm – 100 μm. Thus increased La wt. % loadings resulted in larger catalyst particle sizes which in turn caused larger fiber thicknesses. The RANR was observed to be an actively interactive support that enhances the catalytic performance of the metal nanoparticles on its surface especially with Ni in the synthesis of CNFs over Ni/RANR catalysts. Variation of parametric conditions such as: wt. % loading, reaction temperature, gas flow-rate and reaction time had a significant effect on the morphology, mechanical properties and density of the CNFs grown on the Ni/RANR catalysts. The Ni/RANR catalysts were observed to support the synthesis of twisted CNFs in contrast to the straight chain CNFs synthesised on La/RANR catalysts. The twisted CNFs were synthesized by way of regular oriental nucleation of heptagons and pentagons along the nanofibre body. CNFs growth time was found to be a crucial parameter that can be used to understand the growth process and tailor the artificial properties of the resulting carbon material assembly. There was a threshold time interval over which both growth of fibres was consistent and uniform coverage was achieved. Detailed studies on CNFs synthesised over 1, 8 and 10 wt. % Ni/RANR showed similar trends with time at various temperatures, wt. % loading and flow rates while the 0.5 wt. % Ni/RANR did not catalyse the synthesis of twisted CNFs except a few amorphous carbon deposited on the catalysts. It was also proved that the longer the time, the more graphitic the CNFs. Thus the carbon crystallisation on the Ni/RANR catalyst resulted in the formation of primary amorphous carbon. Then sufficient time of heating transformed the carbon into graphitic material while the catalyst shaped it into its specific twisted morphology. Gold however, presented a different set of trends in its performance as a catalyst supported on RANR. The CVD products which formed over Au/RANR catalysts were observed to vary under different parametric reaction conditions. At low wt. % loading, catalysts catalysed the formation of amorphous carbon material. However, Au/RANR at higher wt. % loading was observed to catalyse the synthesis of both straight and coiled or twisted CNFs. Like La and Ni, Au also supported a tip-growth mechanism as Au particles were also observed on many of the tips of the CNFs as the Au particles were dislodged from the RANR support. Temperature facilitated enhanced sintering and the dislodged Au nanoparticles grew to larger particles of different sizes. The sintered nanoparticles determined the diameters of the CNFs synthesized. The fibres synthesized under different parametric conditions ranged from 10–100 nm in thickness with an average diameter of 50 nm. Detailed studies also showed that both the diameter and length distribution range increased drastically with increase in reaction temperature. The temperature enhanced decomposition of C2H2 and thus more carbon was available for forming the carbon matrix of the CNFs. La and Ni were co-loaded on RANR to form a co-loaded La-Ni/RANR catalyst. Metal wt. % loadings ranging from 1-10 wt. % were achieved. The peaks identifying La may be ascribed to La(OH)3 and La2O3 phases while Ni peaks were due to NiO. The phases reduced to metallic La and Ni to give co-loaded La-Ni/RANR catalysts. Reduction peaks shifted to higher temperatures with an increase in metal nanoparticles wt. % loading. The shift in the peak positions to higher temperatures may be attributed to increased crystallinity of the metal oxides and mainly the increase in the oxygen in the metallic matrix of the catalysts. The Ni nanoparticles were characterised by a spherical morphology with a diameter range of 1–6 nm with an average of 4 nm. The short rod structured nanoparticles were those of La with a diameter range of 2–8 nm and an average of 7 nm. The co-loaded La-Ni/RANR catalysts catalysed the synthesis of CNFs that were a mixture of both straight long stretching nanofibers and twisted or coiled nanofibers. The actual morphology of each individual CNF may have been directly attributed to the surface or end of the La-Ni nanoparticle over which it grew. The straight chain CNFs were catalysed by the La end of the co-loaded La-Ni/RANR catalyst, while the coiled or twisted CNFs, were catalysed by the Ni end of this catalyst. During CVD reaction, the respective metal nanoparticles also dislodged from the support, sintered and gave synthesised fibres with a diameter range of 50–500 nm with an average of 175 nm. It was observed that there were no synergistic effects as a result of co-loading La and Ni on the RANR support. The supported catalysts, La/RANR, Ni/RANR, Au/RANR, and La-Ni/RANR showed exceptional performance as catalysts in the synthesis of CNFs. However, the plain RANR had no catalytic effect observed during the CVD synthesis of SCNMs. TiO2 is well known for its excellent photocatalytic behaviour. Therefore the supported catalysts and the plain RANR were compared as catalysts for photocatalytic degradation of methyl orange (MeO). Detailed studies of the preparation of the supported catalysts revealed that metal loading did not result in the blending of the nanoparticles species into the rutile phase lattice of the RANR. Both PXRD and LRS showed the RANR peaks dominating those of the supported nanoparticles. The supported metal nanoparticles caused slight shifting of the RANR peak positions. The low wt. % supported catalysts did not show any significant photocatalytic activity different from that of pure RANR. A 5–8 wt. % loading was observed to be ideal for the photodegradation of MeO. La/RANR showed the highest photocatalytic performance of all the supported catalysts. However, plain RANR exhibited excellent photocatalytic activity, with degradation efficiency that far exceeded that of the whole range of supported catalysts. The co-loaded La-Ni/RANR was the worst photocatalysts due to a combination of Ni inactivity and a tendency to inhibit the photocatalytic activity of both La and plain RANR. Photoluminescence studies exhibited reduced intensities for the supported catalysts indicating consequential effective prohibition of recombination of the electrons and respective holes. However, La/RANR sufficiently slowed the radiative recombination process of photogenerated electrons and holes in TiO2. This is because La loaded TiO2 has a tendency of expanding the wavelength response range as determined by diffuse reflectance UV-Vis spectroscopy.Item Synthesis of copper nanoparticles contained in mesoporous hollow carbon spheres as potential catalysts for growing helical carbon nanofibers(2017) Magubane, AliceThe aim of this study was to synthesize helical carbon nanofibers with controlled diameter by using copper nanoparticles contained inside hollow carbon sphere. In this work, different methods have been explored to synthesize copper nanoparticles contained inside mesoporous hollow carbon spheres in order to minimize the sintering effect of the copper nanoparticles. Mesoporous hollow carbon spheres were used not only as a support for the copper nanoparticles but to stabilize and disperse these nanoparticles to prevent the formation of aggregates. Mesoporous hollow carbon spheres were synthesized using a hard templating method, in which mesoporous silica spheres or polystyrene spheres were used as a sacrificial template. Carbon nanofibers with different morphologies, including straight and helical fibers were obtained by a chemical vapor deposition method where acetylene was decomposed over copper nanoparticles contained inside mesoporous hollow carbon spheres catalyst at 350 °C. The synthesized carbon nanofibers were grown on the surface of the mesoporous hollow carbon spheres as the methods used to synthesize the catalyst failed to incorporate copper nanoparticles inside the spheres. Differences in the diameter of the straight and helical carbon nanofibers were observed from both catalysts. This supports the important effect of particle size on influencing the shape of the synthesized carbon nanofibers.Item Biosynthesis and characterization of metallic nanoparticles produced by paenibacillus castaneae(2017) Hiebner, Dishon WayneNanomaterials (NMs) have been shown to exhibit unique physical and chemical properties that are highly size and shape-dependent. The ability to control synthesis of nanoparticles (NPs) with particular shapes and sizes can lead to exciting new applications or enhancements of current systems in the fields of optics, electronics, catalytics, biomedicine and biotechnology. Due to increased chemical pollution as well as health concerns, biological synthesis of NMs has quickly emerged as potentially being an eco-friendly, scalable, and clean alternative to chemical and physical synthesis. In this study, the inference that the heavy metal-resistant bacteria, Paenibacillus castaneae, has the propensity to synthesize metal NPs was validated. NP formation was achieved after the exposure of bacterial cell biomass or cell-free extracts (CFE) to excess metal ion precursors in solution. These include lead nitrate and calcium sulphate dehydrate, gold (III) chloride trihydrate and silver nitrate, respectively. All reactions were incubated at 37 °C for 72 h at 200 rpm and observed for a colour change. UV–visible (UV-Vis) spectral scans (200 nm – 900 nm) were measured on a Jasco V-630 UV-Vis spectrophotometer. For scanning electron microscopy (SEM), samples were fixed, dehydrated and loaded onto carbon-coated aluminium stubs. The stubs were then sputter-coated with either Au/Pd or Cr and analysed on the FEI Nova Nanolab 600 FEG-SEM/FIB. Size distribution analysis was done using transmission electron microscopy (TEM) using the FEI Tecnai T12 TEM and Image J software. Powder X-ray diffraction measurements were carried out on a Rigaku Miniflex-II X-ray diffractrometer. Colour changes indicative of the synthesis of PbS, Au and Ag NPs were observed as a white precipitate (PbS), purple (Au) and yellow-brown (Ag) colour, respectively. This was confirmed by absorbance peaks at 325 nm and 550 nm (PbS), 595 nm (Au) and 440 nm (Ag) from UV-Vis analyses. Exposure of P. castaneae biomass and CFE to PbS ions in solution resulted in the production of nanospheres, irregularly-shaped NPs, nanorods, nanowires as well as large nanoflowers. Exposure of P. castaneae biomass to Au3+ ions in solution produced Au nanospheres, nanotriangles, nanohexagons, nanopentagons and nanopolyhedrons. Ag/AgCl NP production occurred using both the P. castaneae biomass and CFE, and resulted in the synthesis of nanospheres only. This is the first report of the biosynthesis of such a diverse set of anisotropic NPs by P. castaneae. It is also the first instance in which anisotropic PbS nanorods and nanowires, 3-D Au nanoprisms as well as “rough” Ag/AgCl nanospheres were bacterially produced. This study serves as an eco-friendly approach for the synthesis of NPs that is a simple yet amenable method for the large-scale commercial production of nanoparticles with technical relevance. This in turn expands the limited knowledge surrounding the biological synthesis of heavy metal NMs.Item Synthesis of catalyst nanoparticles encapsulated in mesoporous carbon spheres and their subsequent use as catalysts for the oxygen reduction reaction(2016) Phago, Evah RamokoneIn the current study, different platinum-hollow carbon sphere catalysts were synthesized for use as electrocatalysts in low temperature fuel cells such as proton exchange membrane fuel cells (PEMFCs). The support material was synthesized via a hard templating method using mesoporous silica (synthesized using a modified Stöber method) as a sacrificial template. In fuel cells, one aim is to ensure that as much platinum as possible is present on a given electrode while keeping the entirety of the catalytic layer as thin as possible (i.e. with the minimum amount of carbon). One approach to achieving this was to make the hollow carbon spheres as small as possible, starting of course with the templating material. It was found that tailoring the molar ratios between the two co-solvents (that is water and ethanol) during Stöber synthesis was the key to achieving particles as small as approximately 150 nm with a uniform shape, size, and significant yields of up to 5.00 g. Another focal point in terms of the template material was achieving a silica structure that consisted of a solid core, and a distinctly mesoporous shell. Two different surfactants were explored in order to fabricate these structures; namely octadecyltrimethoxysilane (C18TMS) and cetyltrimethylammonium bromide (CTAB). It was found that of the two, the C18TMS resulted in more distinctly formed mesoporous silica layers with higher measured specific surface areas. Because the type of support material greatly influences the catalytic behaviour of the loaded catalysts, two different carbonization techniques were explored; namely the bubbling method using toluene as a carbon source, and a nanocasting method where resorcinol formaldehyde (RF) was the carbon source. The toluene-synthesized hollow carbon spheres had advantages over their RF-synthesized counterparts in that they were more thermally stable and had a more graphitic crystalline carbon framework. The RF-synthesized carbon, however, possessed a pseudo-capacitance due to surface carbon-oxygen groups, as well as a higher specific surface area, which resulted in the RF-carbon cyclic voltammetry profile spanning a larger current range in microampere per square centimetre. The effect of the size of the support materials was also explored; comparing 350 nm and 150 nm hollow carbon spheres. Besides the type of carbon, the metal precursor used to synthesize the catalyst nanoparticles was also explored, with either platinum(II)chloride (PtCl2) or platinum(II)acetylacetonate [Pt(acac)2] being used as the platinum source. It is also known that achieving high metal yields using conventional methods is quite difficult, and so an easier, quicker and less wasteful method was also explored; comparing the traditional wet-impregnation (WI) method with a chemical vapour deposition (CVD) method. Ultimately, it was found that platinum loaded on top of small-sized toluene-synthesized hollow carbon spheres using the CVD method and Pt(acac)2 as the metal precursor was the better catalyst in terms of oxygen reduction (determined using linear sweep voltammetry measurements); outperforming even commercial Pt/C catalysts as a result of improved mass transfer afforded by the voided cores of the hollow carbon spheres. The ability of a catalyst to withstand the reaction conditions present in a PEM fuel cell (i.e. oxygen-rich environments) was also considered. The stability of the catalysts was tested using chronoamperometry measurements in an oxygen-saturated perchloric acid solution. It was evident that the platinum loaded on the inner shells of the hollow carbon spheres showed far superior stability to those loaded on the outside surface. This was attributed to the qualities bestowed by the carbon shell around the platinum nanoparticles, protecting said platinum against the consequences of support corrosion due to the oxygenated environment; consequences such as metal sintering and interaction with surrounding carbon supports for example. These encapsulated catalysts, however, displayed a decrease in electrocatalytic activity compared to the catalysts with top-loaded platinum. In conclusion, the study of different platinum-carbon catalysts studied in the current work revealed that (a) loading platinum on top of small sized toluene-synthesized hollow carbon spheres using a CVD method and Pt(acac)2 as a metal precursor resulted in a highly active oxygen reduction catalyst, while (b) loading platinum on the inside surface of the hollow carbon spheres under the dame conditions resulted in a more electrocatalytically stable catalyst.Item Synthesis of Tungsten Oxide Nanostructures by Laser Pyrolysis(2012-02-01) Govender, MalcolmThis dissertation discusses the synthesis method known as laser pyrolysis. The theory on laser pyrolysis has been inferred since 1975, but it is insufficient in predicting the products that can be formed. This is due to the use of a laser, which leads to indecisive reaction pathways from precursor to product. In this work, the laser wavelength and power are varied to initiate a starting point in understanding the complex nature of the laser–precursor interaction, in addition to studying the resulting nanomaterial that is formed by the corresponding laser pyrolysis parameters. The results are justified based on linear and nonlinear optical processes, as well as photophysical and photochemical processes. Experiments to produce tungsten trioxide nanowires were conducted, but similar products could not be achieved, due to the difficulty in emulating ‘sensitive’ variables such as gas pressure and flow rates. However, it was discovered for the first time using this method that six-sided tungsten oxide “stars” can be grown.